Various mechanisms are known in the prior art for use in obtaining variable valve timing and variable valve overlap in an engine whereby to obtain the different advantages obtainable from such varying of valve timing and valve overlap as a function of engine operation.
In United States Pat. No. 4,054,109, entitled "Engine with Variable Valve Overlap" issued to Applicants on Oct. 18, 1977, there is disclosed a variably yieldable load-carrying member or hydraulic valve lifter used in the actuating mechanism of one or both of the intake and exhaust valves of each cylinder of an associated engine.
Although hydraulic valve lifters of the type disclosed in the above-identified U.S. Pat. No. 4,054,109 have been used in engines wherein they were operative to provide the desired function of varying the engine valve opening duration as a function of engine speed, it has now been found that such lifters are not only viscosity sensitive but, at engine speeds above 2200 rpm, in such lifters the amount of lifter collapse would not decrease with increased engine speeds.
Accordingly, it is a primary object of the present invention to provide an improved hydraulic valve lifter for use in an internal combustion engine and which lifter is operative to vary the lift and timing of a cam actuated valve as a function of engine speed, the lifter not being sensitive to changes in the viscosity of oil supplied thereto.
Another object of the present invention is to provide an improved hydraulic valve lifter wherein a thin sharp edge orifice valve disc is used to sense oil flow rate between a hydraulic supply chamber and a pressure chamber within the lifter whereby to regulate, in effect, the volume capacity of the pressure chamber as a function of engine speed essentially independent of oil viscosity.
Still further object of the present invention is to provide an improved hydraulic valve lifter of the above type which is provided with an increased flow area for the flow of oil from the hydraulic supply chamber to the pressure chamber of the lifter whereby the pressure chamber can be filled with oil more readily at high engine speeds.